get a good security for multiplayer game

Question

I made a game using eclipse. I have a server program and a client program. The clients communicate with the server using sockets. If the client starts it sends its username

Answer 1

If it is a server driven multi player game, you can design the client apps to request the server to send back a server only decryptable level key and a client readable level number or key. And on a next level change request, make the clients send the previously received level key (i.e. current) and any other parameters as required to identify a client's session state.

With such server only decryptable level key, clients can't modify it and request with a jump in game levels. Server in turn, will send back with next level key and other related info to the client to continue.

Though there may exist many active client sessions but to validate and authenticate them there should exist such service at servers.